1. Field of the Invention
This invention is in the technical field of energy storage devices, particularly alkaline batteries. More particularly, the present invention is in the technical field of rechargeable batteries employing an iron negative electrode in an alkaline electrolyte, and the formation of such a battery.
2. State of the Art
Iron electrodes have been used in energy storage batteries and other devices for over one hundred years. Iron electrodes are often combined with a nickel positive electrode to form a nickel-iron (Ni—Fe) battery. The Ni—Fe battery is a rechargeable battery having a nickel(III) oxy-hydroxide positive electrode and an iron negative electrode, with an alkaline electrolyte such as potassium hydroxide. The overall cell reaction can be written as:2NiOOH+Fe+2H2O→2Ni(OH)2+Fe(OH)2  (1)
It is a very robust battery which is tolerant of abuse (overcharge, overdischarge, and short-circuiting) and can have a very long life even if so treated. Ni—Fe batteries are often used in backup situations where it can be continuously charged and may last for more than 20 years. However, due to its low specific energy, poor charge retention, and high cost of manufacturing, other types of rechargeable batteries have displaced Ni—Fe in most applications.
The ability of these batteries to survive frequent cycling is due to the low solubility of the reactants in the electrolyte. The formation of metallic iron during charge is slow due to the low solubility of the reaction product ferrous hydroxide. While the slow formation of iron crystals preserves the electrodes, it also limits the high rate performance. Ni—Fe cells are typically charged galvanostatically and should not be charged from a constant voltage supply since they can be damaged by thermal runaway. Thermal runaway occurs due to a drop in cell voltage as gassing begins due to overcharge, raising the cell temperature, increasing current draw from a constant potential source, further increasing the gassing rate and temperature.
As shown in Equation (1), the overall cell reaction does not involve the battery electrolyte; however, alkaline conditions are required for the individual electrode reactions. Therefore, iron-based batteries such as Ni—Fe, Fe-air, and Fe—MnO2 batteries all employ a strong alkaline electrolyte typically of KOH, typically in the range of 30-32% KOH. KOH is preferred due to its higher conductivity and low freezing point. LiOH may be added in cells subject to high temperatures due to its stabilization effects on the nickel electrode, improving its charge acceptance at elevated temperatures.
A known performance issue of iron electrodes is premature passivation of the iron surface. Thus, iron electrodes whose active mass consists of pure iron become passivated after a limited number of cycles. This is apparently due to the formation of iron oxides that form on the electrode surface, inhibiting the charging process.
It is known in the art that the addition of sulfur or sulfides can be added to the iron electrode active mass to inhibit the passivation of the electrode (D. Linden and T. Reddy, Editors, “Handbook of Batteries, Third Edition”, McGraw-Hill, © 2002). Sulfur and/or sulfide addition changes the electrocrystallization kinetics and makes the iron electrode reaction more reversible. Sulfide also is known to absorb on the iron electrode, raising the overpotential for the hydrogen evolution reaction during charging. A disadvantage of the prior art associated with adding sulfur or sulfides to the iron active mass is loss of sulfide over time due to dissolution of sulfide into the electrolyte and resultant oxidation to sulfate, which is ineffective in providing lasting activation of the iron electrode.
The addition of sulfide additives to alkaline electrolyte is similarly known in the art. Particularly, the addition of sulfur content is described in Swedish Pat. No 196,168 which recommends sulfide concentrations on the order of 0.03 to 0.1% of the iron active mass. However, it has been suggested by others that if the local sulfide concentration is too high, the activating effect is actually reversed due to blockage of the active mass. Hence, U.S. Pat. No. 4,250,236A teaches the use of sparingly soluble sulfide compounds whose solubility is at most 10−2 moles per liter. These inventors state that high concentrations of sulfide do not result in any substantial prolongation of the electrode lifetime due to oxidation of sulfide to sulfate, which may precipitate and block the pores of the electrode.
One problem associated with current Ni—Fe batteries is the high rate of self-discharge associated with hydrogen evolution occurring at the charged iron electrode. This occurs due to the fact that the potential for hydrogen evolution is less negative than the potential for the electrode reaction during charge of Fe(OH)2 to Fe. Kinetic effects allow for the charge reaction to proceed, but at low efficiencies.
Another problem associated with present art Ni—Fe batteries is the need for prolonged activation of the cell. As shown in Equation 1, the fully charged negative electrode consists of metallic iron. Hence, as constructed, the iron electrode is in a near fully charged state, existing predominately of metallic iron. In contrast, as constructed the Ni(OH)2 electrodes exist in a fully discharged state in the assembled cell. Thus, the resultant as-constructed cell is largely out of balance with respect to state of charge. Hence, multiple cycles are required to achieve appropriate cell balance by bringing both electrodes to the same state of charge. During initial charge cycles, copious amounts of hydrogen gas are generated at the charged iron electrode during charging of the nickel positive plates.
The formation process is a time-consuming and expensive operation in the manufacture of rechargeable batteries due to the need for expensive battery cycling equipment. As formation time increases, additional capital equipment is required to meet the needs for cycling of multiple batteries, resulting in significant cost. A process reducing formation and activation time, and complexity, would result in significant savings to the battery manufacturer, and would be welcomed by the industry.